The present invention relates to cross-linking of thermoplastic or elastomeric material in the presence of moisture and on the basis of grafting silane on the macro-molecules of the material. Preferably, but not exclusively, the invention relates to enveloping elongated stock, such as electrical cable, conductors, tubes, etc. by such materials, such as olefin-polymerizate or mixed olefin-polymerizates.
The German Pat. application No. 1,794,028 discloses grafting of organofunctional trialcoxysilanes upon polyethylene chains to obtain subsequently cross-linking in the presence of moisture. The grafting is initiated under utilization of radical initiators, such as peroxides, e.g. dicumyl peroxide; di-terbutylperoxide; 1,3-bis (tert.butylperoxi-isopropyl) benzene, etc. Also, azo-compounds such as azo-bisisobutyronitrile can be used as initiator. The grafting of the organo-silane upon the polyethylene molecules can be carried out during mechanical working, whereby free radicals are intermediately produced.
Siloxane cross-linking requires the presence of small amounts of water within the extruded and grafted material. This water is usually provided through diffusion in that e.g. following extrusion the material is placed into hot water. The duration of applying water depends on the thickness the material layer into which water molecules are to diffuse. Also, the water temperature is another factor determining the rate of diffusion. By way of example, a cable to be used for transmitting 1 kilovolts may be provided with a polyethylene insulation of about 1.3 mm thickness. When using boiling water, cross-linking is completed to a satisfactory degree, if the cable is left in the water for about 30 min. The duration of water application is 10 to 20 fold longer when the water temperature is only 80.degree. C.